Many electrical appliances use common line current which is supplied from electrical outlet sockets. The sockets accept a pair of flat blade-type prongs, each rectangular in cross section. The standard prongs are 1/4 inch wide, slightly less than 1/16 inch thick, and almost 3/4 of a inch long. The two prongs are set 1/2 inch apart on centers. Usually a hole for a third, grounding prong is provided. The grounding prong is usually round in cross section, instead of rectangular.
If the appliance is electrically isolated from the line current, as by a transformer or double insulation, then the third grounding prong is not necessary and only the two flat prongs need be provided and inserted into the socket for electrical powering of the appliance. Often the transformer is packaged in a small plastic box or housing, with low voltage wires (from the low side of the transformer) running from the box to the appliance proper. The transformer housing, rather than a jack or "plug" proper, then is placed directly against the socket when the prongs are inserted to power the appliance.
Such transformer housings are almost always molded of rubber or plastic insulating materials. The prongs need to be fastened to the box or housing in such a way that they are firmly held and that electrical contact path can be made to them inside the housing, after which the (usually two) mating parts of the housing are closed.
In the prior art, the conventional method of fastening prongs to transformer and other plug-type housings has been to mold them directly into the plastic. The housing mold has been provided with slots for accepting the prongs and holding them during the plastic injection. Because the prongs must protrude on both the outside and the inside of the housing for electrical connection, both the male and female halves of the mold required slots.
This method has several drawbacks. First, the mold is more expensive because of narrow slots that must be machined into it, and the provisions for inserting the prongs (if inserted from outside the mold). Second, the prong and its hole form a leakage point for molten plastic, and can result in flash that covers the prong surfaces to interfere with electrical contact, spoil the housing appearance, and, during soldering of leads to the inside ends of the prongs, burn and give off fumes. Third, the molding operation is more difficult and slower because the mold must be opened wide to release the housing with its prongs, the prongs require time for insertion, and so on. Fourth, the differential temperature expansion coefficients of metal prongs and plastic housing may cause problems while injecting hot plastic or during cooling.